Method of combustion and a combustion plant in which absorbent is regenerated

ABSTRACT

A method of combustion and a combustion plant in which absorbent is regenerated is described herein. During the combustion of a fuel in a combustion chamber enclosing a fluidized bed, a fuel and an absorbent are supplied to the fluidized bed. The combustion gases generated during the combustion are collected and purified in a separating member by separation of solid material from the combustion gases. The separated solid material is recirculated to the fluidized bed through a channel, and a gaseous medium is supplied in a controlled manner to the separated solid material present in the channel in order to displace the combustion gases and provide a chemical reaction.

The present invention refers to a method of combustion of a fuel in acombustion chamber enclosing a fluidized bed, comprising the steps of:supplying an oxygen-containing gas to the bed from beneath; supplying afuel and an absorbent to the bed; collecting combustion gases formedduring the combustion; purifying the combustion gases by separatingsolid material therefrom; and recirculating the solid material separatedto the combustion chamber through a channel. Furthermore, the inventionrefers to a combustion plant comprising: a combustion chamber, which isprovided to enclose a fluidized bed and in which a combustion of a fuelis intended to be performed while forming combustion gases and duringthe supply of an absorbent; and a purification device for purifying saidcombustion gases, said purification device comprising a separatingmember, arranged to separate particulate material from said combustiongases, and a channel, connecting the separating member and thecombustion chamber and being arranged to recirculate the materialseparated to the combustion chamber.

It is known to combust different fuels in a bed of particulate,incombustible material, which bed is supplied with combustion air frombeneath through nozzles in such a manner that the bed becomes fluidized.There is a difference between different types of such combustion in afluidized bed, which operate according to different principles and underdifferent conditions. Firstly, there is a difference between anatmospheric bed and a pressurized bed. In comparison with an atmosphericbed, a pressurized fluidized bed is characterized by a small plant sizein relation to the effect produced, by a high efficiency, and in thatthe combustion occurs under advantageous conditions from anenvironmental and economical point of view. A pressurized bed may have alarger height than atmospheric bed since one may operate with greaterpressure drops. Among the atmospheric beds, so-called circulating bedsare frequently used, in which the bed material is permitted to circulatethrough a separating device in order to be recirculated to the bed. Thisenables unburnt fuel to be recirculated, which improves the efficiencyof the combustion, as well as the absorbent material not used forabsorption of contaminants, sulfur, which decreases the discharge fromthe combustion. However, such circulating beds operate with relativelyhigh fluidizing velocities, in typical cases in the order of 5-12 m/s.Fluidizing velocity is the velocity that the gas would have had if itwould have flowed through the combustion chamber without the pressure ofparticles. This high velocity causes problems with erosion of forinstance the steam tube arrangement provided in bed in such a way thatthe lifetime thereof significantly decreases. Furthermore, one maydiscern the so-called bubbling beds in which the fluidizing velocity isrelatively low, in typical cases between 0.5 and 2 m/s. Such a bed isrelatively well defined in a vertical direction and there is formed aspace, called a freeboard, in the combustion chamber above the bed. Inthis freeboard a relatively small amount of dust particles are presentin comparison with a circulating bed but there is essentially nopressure drop across the freeboard.

In recent time some have tried to provide a certain circulation inpressurized beds by supplying the combustion cases leaving thecombustion chamber to a cyclone for separation of solid material, whichis recirculated to the combustion chamber. In order to obtain completelythe desired effect concerning the degree of utilization of the absorbentand the combustion efficiency by the recirculation, the solid materialshould be supplied at the bottom of the fluidized bed. This means thatone has to overcome the pressure drop which is present in the bed and inthe cyclone, in typical cases about 0.5 bars.

In order to overcome this pressure drop it has been suggested to providea dosing device, for example of a cell feeding type, at the end of arecirculating pipe provided preferably vertically and connecting thecyclone to a combustion chamber. The dosing device may comprise arotatable shutter provided on the pipe and having a weight which innormal cases keeps the shutter in a closed position. When the amount ofmaterial in the pipe is sufficient the weight thereof will overcome theweight of the shutter which means that the shutter is opened and thematerial is discharged. Such a device leads to an intermittentrecirculation of solid material. However, such devices do not functionin the way intended in the environment of a fluidized bed due to themovements occurring in the bed and the forces caused by these movements.Furthermore, such devices are rapidly destroyed due to the aggressive,erosive and corrosive environment.

Another solution is an L-valve located in the bed and having a verticalportion in which a column of material is built up. In order to provide aflow of material through the channel such a device requires that gas isinjected in the lower portion of the L-valve and in order to providestability it is necessary to continuously measure the height of thecolumn of material, which is very difficult, if not impossible, in theactual environment.

SE-B-460 148 suggests another way of overcoming this pressure drop.SE-B-460 148 discloses a combustion plant having a combustion chamberenclosing a pressurized fluidized bed for the combustion of a fuel whileforming combustion gases. Furthermore, the plant comprises apurification of said combustion gases in several stages. In the stageparticulate material is separated by means of a cyclone from thecombustion gases and supplied to a collection chamber beneath thecyclone. Via a horizontal recirculating channel, the collected dustparticles are fed back to the combustion chamber in order to improve theuse of unburnt fuel and absorbent material. The recirculation isaccomplished by means of an air driven ejector blowing the material intothe combustion chamber. However, such an air injection is veryexpensive. The increase in the absorbent utilization and the combustionefficiency is lost for the compressor providing primary air to theejector. In addition, this method leads to erosion.

It should be noted that the recirculation of solid material separatedfrom the combustion gases means that the recirculated fine part mayprovide as much as 10-40% of the mass of the bed, which stronglyinfluences the heat transfer coefficient to the tubes located in thebed. The fine part is comprised of particles having a largest diameterof about 300-400 μm and an average particle diameter of about 50-150 μm.

U.S. Pat. No. 4,021,184 discloses a combustion plant developed for thecombustion of waste material. The plant comprises a combustion chamberfor a recirculating fluidized bed. The bed disclosed in this document isnot pressurized but the plant operates at atmospheric pressure and is ofa diluted type (dilute phase fluidized bed), i.e. the fluidized bedfills up the whole combustion chamber. Such a type of bed means that avery large part of the solid, hot bed material will be transported outfrom the combustion chamber together with the combustion gases formedduring the combustion.

Therefore, it is suggested that cyclones for separating dust particlesfrom these gases are provided at the outlet of the combustion chamberand that the separated, hot dust particles are recirculated to thecombustion chamber via conduit pipes connecting the cyclones with thecombustion chamber. In such a manner it is possible to recover the heatenergy in the dust particles leaving the combustion chamber. Thus, arecirculation may be obtained due to the low pressure drop across thebed, i.e. the whole combustion chamber. In addition, the valve mentioned(trickle valve) in the end of the conduit pipe is probably necessary.

EP-B-176 293 discloses another combustion plant having a combustionchamber which encloses a fluidized bed and in which combustion of a fuelis intended to be performed while forming combustion gases. The bed isof a bubbling type but the combustion chamber operates at atmosphericpressure. Furthermore, the plant comprises a cyclone for separatingparticulate material from the combustion gases and provided above thecombustion chamber. The particulate material separated is conducted viaa pipe back into the bed by letting the material simply fall freelythrough the pipe. This is possible since the bed disclosed in thisdocument has a relatively low height, about 1 m. Thereby, also thepressure drop is relatively small.

U.S. Pat. No. 4,103,646 discloses a plant comprising two combustionchambers, the first combustion chamber having a fast fluidized bed,i.e., the fluidizing velocity is between 7 and 10 m/s, and the secondcombustion chamber having a “slow”, bubbling fluidized bed. Thecombustion gases formed in the first combustion chamber are conducted toa cyclone, where solid material is separated and fed to the secondcombustion chamber. In the bottom of the second combustion chamber thereis a discharge channel for solid material which by means of airinjection then is recirculated to the first fast combustion chamber.

By recirculating the separated material to the bed a higher degree ofutilization of the absorbent supplied to the combustion chamber isobtained since its time of presence in the process may be prolonged.

SUMMARY OF THE INVENTION

The object of the present invention is to provide a method and acombustion plant by which the discharge of undesirable substances may bereduced. In particular, the invention aims at a higher degree ofutilization of the absorbent supplied to the combustion for absorbingundesirable substances.

This object is obtained by the method initially defined andcharacterized in that a gaseous medium in a controlled manner issupplied to the separated solid material present in the channel in orderto displace the combustion gases and provide a chemical reaction. Duringheating of the absorbent, which for instance may be present in the formof a natural lime-containing substance such as limestone or dolomite, inthe combustion chamber carbon dioxide evaporates from CaCO₃, therebyforming quick lime. This has very good ability of binding sulphur,released during the combustion, thereby forming gypsum. However, inorder to be able to obtain large quantities of quick lime, the partialpressure of carbon dioxide has to be low in the gas surrounding thelime-containing absorbent. By the supply in a controlled manner, inaccordance with the present invention of a gaseous medium to theseparated solid material present in the channel, the partial pressure ofthe carbon dioxide may be lowered in this channel in such a manner thatthe absorbent present in the separated material reacts and forms quicklime. Thus, by means of the present invention, the ability of theabsorbent to absorb undesirable substances such as sulphur is improvedto a high extent. Consequently, by the method according to the inventiona reduction of the discharges of undesirable substances may be obtained.The method according to the invention also is suitable for absorption offor instance alkali compounds, chlorine compounds, chlorine and heavymetals.

According to an embodiment of the invention, the gaseous medium ispreheated prior to being supplied to the separated material in thechannel. In such a manner, the environmental capacity of the plant maybe further improved since the ability of the absorbent to evaporatecarbon dioxide and absorb sulphur or other undesirable substancesincreases as the temperature increases. Preferably, the gaseous mediumis preheated to a temperature of about 600-900° C., preferably 700-870°C.

According to a further embodiment of the invention, the gaseous mediumcomprises air. This is a very advantageous embodiment of the invention,which permits a simple structure of the gas supply according to theinvention. The gaseous medium may also comprise steam. Water formscalcium hydroxide together with quick lime. Furthermore, the gaseousmedium may comprise nitrogen or a combination of at least two of saidsubstances.

According to a further embodiment of the invention, the gaseous mediumis supplied to the channel at a plurality of levels of different height.Thereby the degree of utilization of the absorbent may be furtherimproved. In such a preferred application, air and/or nitrogen issupplied to the channel at a higher level and steam is supplied to thechannel at a lower level.

According to a further embodiment of the invention, the absorbentcomprises a lime-containing material, for example limestone or dolomite.

According to a further embodiment of the invention, the separatedmaterial is supplied to a channel in such a manner that a column ofmaterial is formed therein and that the column of material merely due toits weight recirculates the separated material in a continues flowthrough a passage having a constant opening area and being provided inthe lower portion of the channel. Thus, the discharge of the materialinto the combustion chamber is performed merely by the weight of thecolumn of material and without any influence from outside by previouslyused auxiliary means such as ejectors or the like. The operation of theplant ensures that the column of material is filled from above throughthe separating member. Advantageously, the height of the column ofmaterial so formed exceeds the height of the bed. Furthermore, the gasfrom beneath is prevented from entering the channel. In such a manner nofluidization of the material present in the channel may take place andthe recirculation of the material will not be hindered.

The object mentioned above is also obtained by the combustion plantinitially defined and characterized by at least one member connected tothe channel and arranged to supply a gaseous medium to the separatedmaterial present in the channel.

There are several preferred embodiments of the combustion plant of thepresent invention.

Advantageously, heating members are arranged to preheat the gaseousmedium. The heating member may comprise a heat exchanger provided in thecombustion chamber.

According to a preferred embodiment, the channel comprises an enlargedportion arranged to provide an increased volume to the column ofmaterial. By such an enlarged portion, the time of presence of theseparated material in the column of material may be further prolonged.

According to a further preferred embodiment of the invention, the gassupply member comprises at least one gas feeding device provided in wallof the channel. Advantageously, this device may comprise a cylinderprovided around the channel and having an upper and lower limiting wallso that a closed annular space is formed between the channel and thecylinder and the wall of the channel may comprise a passage between theinterior of the channel and the annular space.

According to a further advantageous embodiment, the channel comprisespassive means which are provided in such a manner that a column ofmaterial is formed in the channel during the operation of the combustionplant and which are forming, in the lower part of the channel, a passagewhich permits that the weight of the column of material discharges thematerial therethrough in a continues flow. The flow area of the passagemay be constant. Thus, merely the weight of the column of material willprovide a continuous and uniform recirculation of separated solidparticulate material into the combustion chamber. Since therecirculating device according to the invention comprises passive meansnot requiring any compressor or other drive means for overcoming thepressure difference and feed out the material from the column ofmaterial, the cost of the device is favourable with respect tomanufacture as well as operation. Furthermore, the problems of erosionin connection with ejector feeding of the material are avoided. Thus,since the recirculating channel does not comprise any movable structuralelements, the reliability thereof is very high.

Furthermore, the passive means may comprise a surface which is providedat the lower end of the channel and seen from beneath covers at least alarger part of the cross-section area of the channel. This surface mayform an angle of inclination to a vertical axis which amounts to about20-90°, preferably 21-39°. By such a sloping surface, gas is preventedfrom entering the channel and at the same time the surface facilitatesthe recirculation of the material into the combustion chamber. Thesurface having such a favourable angle of inclination will function assliding surface or a type of chute for the material.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained by means of different embodiments,defined by way of example, and with reference to the drawings attached.In the drawings:

FIG. 1 discloses schematically a PFBC-power plant having a combined gasand steam cycle (the latter not disclosed);

FIGS. 2-5 disclose different embodiments of a combustion chamber and arecirculation channel of the power plant according to the invention forsolid material separated from the combustion gases; and

FIGS. 6-12 disclose different embodiments of the recirculation channel.

DETAILED DESCRIPTION OF DIFFERENT EMBODIMENTS

The invention will now be explained with a reference to a so calledPFBC-power plant (pressurized, fluidized bed combustion). However, itshould be noted that the invention also is applicable to other types ofplants, in particular combustion plants without power production. APFBC-power plant, i.e. a plant for the combustion of particulate fuel ina pressurized, fluidized bed, is schematically disclosed in FIG. 1. Theplant comprises a combustion chamber 1 being housed in a pressure vessel2, having a volume in the order of 10⁴ m³ and which may be pressurizedto for example between 7 and 30 bars (abs). Compressed oxygen-containinggas, in the example disclosed air, is supplied to the pressure vessel 2at 3 for pressurizing the combustion chamber 1 and for fluidizing a bed4 in the combustion chamber 1. The compressed air is supplied to thecombustion chamber 1 via schematically indicated fluidizing nozzles 5being provided in the bottom of the combustion chamber 1 for fluidizingthe bed 4 enclosed in the combustion chamber 1. The air is supplied insuch a manner that a fluidizing velocity of about 0.5-2.0 m/s isobtained. The bed 4 is of a bubbling type and may have a height h beingabout 2-6 m. It comprises a non-combustible, particulate bed material,particulate absorbent and a particulate fuel. The particle size of thebed material not circulating, the absorbent and the fuel is betweenabout 0.5 and 7 mm. The bed material comprises for example ashes and/orsand and the absorbent a lime containing material, for example dolomiteor limestone for absorption of undesirable substances released duringthe combustion, e.g. sulphur, chlorine, heavy metals, alkali compoundsetc. The fuel is supplied in such a quantity that it forms about 1% ofthe bed. By fuel is meant all fuels which may burn such as for examplepit coal, brown coal, coke, peat, biofuel, oil shale, pet coke, waste,oils, hydrogen gas and other gases, etc. The bed material, the absorbentand the fuel are supplied to the bed 4, via a conduit 6 schematicallydisclosed. The fuel is combusted in the fluidizing air supplied to thebed 4 while forming combustion gases. These are collected in a space 7located above the bubbling bed 4, a so called freeboard, and are thenconducted via a channel 8 to a separating member 9, in the exampledisclosed a cyclone. From there the combustion gases are conductedfurther to further purification devices, which are disclosedschematically at 10 and which for example may comprise cyclones providedin several stages. Thereafter, the combustion gases are conductedfurther via for example a high temperature filter 11 to a gas turbine 12which in the example disclosed comprises a high pressure stage 13 and alow pressure stage 14. The high pressure turbine 13 is provided on thesame shaft as the high pressure compressor 15 and a generator 16 whichin this manner is driven by the high pressure turbine for producingelectrical energy. The high pressure compressor 15 delivers compressedair to the combustion chamber 1 via the conduit 17.

The combustion gases expanded in the high pressure turbine 13 areconducted to a low pressure turbine 14. The combustion gases leaving thelow pressure turbine 14 still comprise energy which may be recovered inan economizer 18. The low pressure turbine 14 is provided on the sameshaft as the low pressure compressor 19 which is supplied with air fromthe atmosphere via a filter 20. The low pressure compressor 19 is thusdriven by the low pressure turbine 14 and provides from its outlet thehigh pressure compressor 15 with air which has been compressed in afirst stage. Between the low pressure compressor 19 and the highpressure compressor 15 an intermediate cooler 21 is provided forlowering the temperature of the air supplied to the inlet of the highpressure compressor 15.

Furthermore, the power plant comprises a steam turbine side, which isnot disclosed, but indicated by the arrangement in a form of a tube set22, which is submerged in the fluidized bed 4. In the tube set 22 wateris circulated evaporated and superheated by heat-exchange between thetubes and the bed material for receiving the heat produced by thecombustion performed in the bed 4.

In the cyclone 9 provided in connection to the combustion chamber andalso called zero step cyclone, solid particulate material is separatedfrom the combustion gases. These solid particulate material comprises onone hand bed material and ashes but also unburnt fuel and absorbent. Itis therefore desirable to recirculate this unused material to the bed 4for, if possible, combust unburnt fuel and utilize unused absorbent.This recirculation is performed by a recirculation device comprising achannel 23. The channel 23 shall be configured in such a manner that acolumn 24 of material is formed in the channel 23 during the operationof the plant. The column 24 of material so formed shall have a height h′exceeding the height h of the bed 4. Due to this height difference thegravity will influence the material in the column 24 of material in sucha manner that this is fed continuously downwardly into the combustionchamber 1 and in the examples disclosed downwardly into the bed 4 underthe tube set 22. This height difference may be provided by a variety ofdifferent embodiments of the channel 23. The channel 23 may have anarbitrary cross-section, for instance circular, oval elliptic,rectangular, polygonal etc. In FIG. 1 the recirculation device comprisesan inclined wall 25 in the lowest portion of the channel 23, which inco-operation with the channel 23 forms a passage with a constant flowarea. Thus, the orifice of the channel 23 is formed by the lowest edgeof the inclined wall 25 and an edge of the channel 23 thereabove. Theinclined wall may have an angle v of inclination in relation to thevertical axis which amounts to about 20 to 90°, i.e. in the extreme caseis perpendicular to said vertical axis. A preferred angle v ofinclination is between about 21 and 39°. The inclined wall 25 preventsthe gas flowing upwardly from the nozzles 5 from entering the channeland functions as sliding surface for the material flowing downwardly. Insuch a manner a column of material of the downwardly flowing material isformed. In order to reduce the recirculation velocity the opening areaof the orifice may be less than the cross-section area of the channel23. It should be noted that the orifice in the example disclosed in FIG.1 is completely located in an essentially vertical plane. Since merelysmall quantities of the combustion air flowing upwardly thus may enterthe channel 23 no fluidizing of the material present in the channel 23will take place.

FIGS. 2-5 disclose other embodiments of the recirculation channel 23 andthe separating member 9. It should be noted that elements having acorresponding function have been provided with the same reference signsin the different embodiments.

The recirculation device disclosed in FIG. 2 comprises a relatively softcurve 26 in the lower part of the channel 23. The orifice is also inthis example formed by cutting the channel 23 in an essentially verticalplane. A lower tangential plane of the curve 26 at the end of thechannel is inclined in relation to a vertical axis by the angle v whichmay have the same value as in the example disclosed in FIG. 1. The curve26 disclosed forms a passage which will prevent gas flowing upwardlyfrom entering the channel 23 and function as a sliding surface for thematerial flowing downwardly. In order to reduce the recirculationvelocity of the material the channel 23 may have a smaller cross-sectionarea at the curve 26 than upstream thereof. In addition, the cyclone 9disclosed in FIG. 2 is completely enclosed in the combustion chamber 1.

The recirculation device disclosed in FIG. 3 comprises a channel 23which extends outside the combustion chamber 1 and in a direction whichforms an angle v to a vertical axis. The channel 23 extends through apassage in the wall of the combustion chamber 1, which passage forms theorifice of the channel 23. The angle v may for example be between 10 and50°, preferably between 21 and 39°. By means of such a slopingrecirculation channel 23 the quantity of gas flowing upwardly in thechannel is reduced, resulting in the formation of a column 24 ofmaterial extending upwardly above the bed 4. Merely the weight of thiscolumn 24 of material ensures an equal and continuous discharge of thesolid material separated. In order to reduce the recirculation velocityof the solid material flowing downwardly, also in this case thecross-section area at said passage, i.e. in the proximity of the orificeof the channel 23, may be less than at a higher position of the channel23. The cyclone 9 is in this example located completely outside thecombustion chamber 1 and is connected therewith via the schematicallydisclosed pipe conduit 8. Although the orifice of the channel 23 in FIG.3 is located at the same height as the tube set 22, it should be notedthat the orifice disclosed in FIG. 3 may be located below or above thelevel of the tube set 22.

FIG. 4 discloses another variant of a recirculation device having achannel 23 extending essentially vertically. In this case therecirculation device comprises a portion 27 of the channel 23 slopingdownwardly, which reduces the quantity of gas flowing upwardly in thechannel 23 and functions as a sliding surface for the solid particulatematerial flowing downwardly. The portion 27 forms a passage having aflow area which has such a dimension that a column 24 of material isformed and having a height h′ exceeding the height h of the bed 4. Thecyclone 9 disclosed in FIG. 4 is enclosed in the combustion chamber 1and located in its upper part, i.e. the freeboard 7.

FIG. 5 discloses another variant of a recirculation device having achannel 23 similar to the one in FIG. 2 but having an orifice in thefreeboard 7 of the combustion chamber 1.

FIGS. 6-12 disclose further variants of the recirculation deviceaccording to the invention. In FIG. 6 this device comprises a channel 23similar to the one in FIG. 1 but the lower plate 25 extends essentiallyperpendicular to a vertical axis. This embodiment is especially simplefrom a manufacturing point of view. There will be formed an accumulation29 of material flowing downwardly in the corner formed by the plate 25and the channel 23. This accumulation will function as a sliding surfacefor the material flowing downwardly. The channel 23 disclosed in FIG. 7comprises a portion 27 similar to the one in FIG. 4 but the lower partof the portion 27 sloping with the angle v is prolonged in the directionof the outflowing material in relation to the upper part of the slopingportion 27. In such a manner the orifice of the channel 23 will have anangle a of inclination in relation to a vertical axis. By thisembodiment the quantity of gas flowing upwardly in the channel 23 isreduced. The channel disclosed in FIG. 8 is similar to the one disclosedin FIG. 1 but the plate 25 sloping with the angle v is shortened in sucha manner that seen from beneath the plate does not cover the wholecross-section area of the channel 23. Thus, the orifice of the channel23 forms an angle b to a vertical axis. By such an embodiment most ofthe gas flowing upwardly will certainly be prevented from entering thechannel 23 but a part thereof is permitted to mix up with the column 24of material. This may be desirable in certain applications when onewishes a gas mixture in the material separated. In FIG. 9 the channel 23comprises a plate 30 being fixed in the channel 23 in such a manner thatan essentially peripheral opening is formed between the plate 30 and thechannel 23. The plate 30 may be fixed by means of a number of barlikerods schematically disclosed at 31. It should be noted that the plate 30also may be provided sloping with an angle v in relation to a verticalaxis. The recirculation device disclosed in FIG. 10 comprises adownwardly completely open channel 23 having an orifice precisely abovea bottom plate 32 of the combustion chamber 1. In the portion 33 of thebottom plate 32 being located below the channel 23 there are nofluidizing nozzles 5 which otherwise are provided over essentially thewhole surface of the bottom plate 32. In such a manner no gas flowingupwardly from the nozzles 5 may enter the channel 23 and causing afluidization of the material present therein. Thereby, a column 24 ofmaterial may be built up and an uniform and continuous discharge ofmaterial to the lower part of the bed is obtained. The material sodischarged will thereafter be brought upwardly in the bed due to the gasflowing upwardly from the nozzles 5. FIG. 11 discloses a recirculationdevice similar to the one in FIG. 10 but the portion 33 provided in thebottom plate 32 and having no fluidizing nozzles 5 is raised in relationto the other surface of the bottom plate 32. The recirculation devicedisclosed in FIG. 12 comprises the channel 23 having a funnel-shapedconical extension 34 being open downwardly. In this extension 34 a coneis provided by means of one or more attachment plates 36. The extension34 and the cone 35 form a cone angle v in relation to the vertical axis.This angle v is, as these in the preceding example, between 20 and 90°,preferably between 21 and 39°.

Furthermore, the power plant comprises a conduit 40 for the supply of agaseous medium to the standing column 24 of material in the channel 23.In the embodiment disclosed in FIG. 1, the conduit 40 is provided tosupply air from the compressor 15. Furthermore, the conduit 40 disclosedin FIG. 1 comprises two branch conduits 41 and 42 for feeding thegaseous medium at different heights in the standing column 24 ofmaterial. Furthermore, the conduit 40 passes a heat exchanger member 43on its way to the column 24 of material. The heat exchanger member 43 isprovided in the combustion chamber 1, for example in the freeboard 7. Insuch a manner the gaseous medium may be preheated to a suitabletemperature of about 600-900° C., preferably about 700-800° C. Thesupply of a gaseous medium is regulated by means of a regulating valve44 which may be connected to the overall control system of the plant(not disclosed). The branch conduits 41 and 42 have an orifice in acylinder 45 forming an annular space 46 between the cylinder 45 and thechannel 23. The space 46 is delimited upwardly and downwardly by wallmembers 47 and is thus closed against the combustion chamber 1. Betweenthe annular space 46 and the channel 23 there is at least one passage 48provided in the lower part of the space 46 for a throughflow into thechannel 23 of the gas supplied via the conduit 40. Furthermore, at leastone of the branch conduits 41 and 42 may comprise a throttling 49. As isdisclosed in FIG. 2, the gaseous medium supplied to the standing column24 of material may also be supplied from a separate external source 50.This source 50 may comprise a suitable gas, for example air, nitrogen orargon. As is disclosed in FIG. 3, the gaseous medium may be a mixture ofthe combustion air supplied by the compressor 15 via the conduit 40 andsuch an additional medium which is supplied from a separate source 50via the conduit 51. It should be noted that the additional medium alsomay comprise steam. Moreover, FIG. 2 discloses that the recirculatingchannel 23 may be provided with an additional space for the separatedmaterial present in the column 24 of material. This additional space maybe formed by, as disclosed, an enlargement 52 of the channel 23. In sucha manner, the time of presence for the separated material in the channel23 is further increased. In FIG. 4, it is disclosed how combustion airmay be supplied to the channel 23 in its upper part via the conduit 40and steam to the lower part of the channel 23 via the conduit 53.

The function of the power plant according to the invention will now beexplained more closely with reference to FIG. 1 disclosing the supply ofcombustion air from the compressor 15 via the conduit 40 to the heatexchanger 43 provided in the freeboard 7 of the combustion chamber 1.Thereby, the combustion air will have a temperature of about ca 850° C.This combustion air is then supplied to the column 24 of material viathe branch conduits 41 and 42. Due to this air supply, the partialpressure of air in the column 24 of material will increase, therebydecreasing the partial pressure of carbon dioxide therein. In such amanner appropriate conditions for the lime, CaCO₃, present in the column24 of material to release carbon dioxide and form quick lime, CaO, havebeen created by heating the air and increasing the air pressure. Thequick lime formed has a good ability to bind sulphur dioxide formedduring the combustion in the combustion chamber 1, thereby forminggypsum, CaSO₄.

Furthermore, the power plant according to the invention may be providedwith equipment for a so called freeboard combustion, see FIGS. 2 and 4.This means that a complementary fuel is injected via the supply nozzles54 into the freeboard 7 of the combustion chamber 1. The complementaryfuel may be oil, gas or any other volatile substance, or advantageouslythe same particulate fuel as is combusted in the bed 4. Due to therecirculation via the channel 23 the undesired particles formed duringthe combustion of the complementary fuel will be brought into contactwith the absorbent. Thereby, the temperature of the material present inthe channel 23, in particular at low load, may be raised by means of thefreeboard combustion.

The present invention is not limited to the embodiments disclosed abovebut may be varied and modified within a scope of the following claims.It should be noted that the channel portions disclosed in FIGS. 6-12 maybe provided with some or several of the gas supply member disclosed inFIGS. 1-5.

In certain applications of the present invention, it might beadvantageous to provide two or more separating members 9 in a parallelconfiguration with each other. Each separating member 9 is in this casepreferably provided with a recirculation channel 23. Such a parallelconfiguration may for example be necessary in order to achieve anappropriate separation efficiency.

What is claimed is:
 1. A method of combustion of a fuel, comprising thesteps of: combusting the fuel in a combustion chamber enclosing apressurized fluidized bubbling bed to form combustion gases in thecombustion chamber; supplying an oxygen-containing gas to the bed frombeneath; supplying a lime-containing absorbent to the bed; collectingcombustion gases formed during the combusting step; purifying thecombustion gases by separating solid material therefrom; andrecirculating the separated solid material to the combustion chamberthrough a channel, wherein the channel has an orifice connected to thebed, and a gaseous medium is supplied in a controlled manner to theseparated solid material present in the channel to displace thecombustion gases therein so that the partial pressure of carbon dioxidein the channel is lowered, thereby permitting a reaction of theabsorbent in the channel to quick lime.
 2. A method according to claim1, wherein the combustion chamber comprises a space above the bed, andthe channel is located in the space and in the bed.
 3. A methodaccording to claim 1, wherein the gaseous medium is preheated prior tobeing supplied to the separated material in the channel.
 4. A methodaccording to claim 3, wherein the gaseous medium is preheated to atemperature of about 600° C. to 900° C.
 5. A method according to claim1, wherein the gaseous medium comprises air.
 6. A method according toclaim 1, wherein the gaseous medium comprises steam.
 7. A methodaccording to claim 1, wherein the gaseous medium comprises nitrogen. 8.A method according to claim 1, wherein the gaseous medium comprises acombination of at least two gases selected from the group consisting ofair, nitrogen, and steam.
 9. A method according to claim 1, wherein thegaseous medium is supplied to the channel at a plurality of levels. 10.A method according to claim 9, wherein air and/or nitrogen are/issupplied to the channel at a higher level and steam is supplied to thechannel at a lower level.
 11. A method according to claim 1, wherein thelength of time the material resides in the channel is increased by anenlarged portion of the channel.
 12. A method according to claim 1,wherein the absorbent comprises limestone or dolomite.
 13. A methodaccording to claim 1, wherein the material is supplied to the channel insuch a manner that the weight of a column of material recirculates thematerial in a continuous flow through a passage having a constantopening area and being provided in a lower portion of the channel.
 14. Amethod according to claim 13, wherein the height of the column ofmaterial exceeds the height of the bed.
 15. A method according to claim13, wherein the oxygen-containing gas provided to the bed from beneathis prevented from entering the channel.
 16. A combustion plantcomprising: a combustion chamber enclosing a pressurized fluidizedbubbling bed, wherein a fuel is combusted and forms combustion gaseswithin said combustion chamber while a lime-containing absorbent issupplied to the bed; a purification device for purifying the combustiongases, said purification device having a separating member forseparating particulate material from the combustion gases, and a channelconnecting the separating member and said combustion chamber forrecirculating the separated material to said combustion chamber, whereinthe channel has an orifice connected to the bed; at least one gas supplymember connected to the channel for supplying a gaseous medium to theseparated material present in the channel to displace the combustiongases in the channel, lower the partial pressure of carbon dioxide, andenable reaction of the absorbent; and a heat exchanger provided in saidcombustion chamber for preheating the gaseous medium supplied to said atleast one gas supply member.
 17. A combustion plant according to claim16, wherein said combustion chamber comprises a space above the bed, thechannel being located in the space and in the bed.
 18. A combustionplant according to claim 16, wherein said at least one gas supply membersupplies air to the material present in the channel.
 19. A combustionplant according to claim 16, wherein said at least one gas supply membercomprises at least one gas feeding device provided in a wall of thechannel.
 20. A combustion plant according to claim 19, wherein the atleast one gas feeding device comprises a cylinder provided around thechannel and having a limiting wall so that a closed annular space isformed between the channel and the cylinder, and the wall of the channelcomprises a passage between the interior of the channel and the closedannular space.
 21. A combustion plant according to claim 16, whereinsaid at least one gas supply member comprises a plurality of gas feedingdevices supplying the gaseous medium to the channel at a plurality oflevels.
 22. A combustion plant according to claim 16, wherein thechannel comprises an enlarged portion that increases the volume ofmaterial held in the channel.
 23. A combustion plant according to claim16, wherein the channel comprises passive portions, and a column ofmaterial is formed in the channel during the operation of the combustionplant, the passive portions forming a passage in the lower part of thechannel that permits the weight of the column of material to dischargethe material therethrough in a continuous flow.
 24. A combustion plantaccording to claim 23, wherein the passage has a constant flow area. 25.A combustion plant according to claim 23, wherein the column of materialformed during the operation of the combustion plant has a heightexceeding the height of the bed in said combustion chamber.
 26. Acombustion plant according to claim 23, wherein the passive portionsprevent a gas from beneath from entering the channel.
 27. A combustionplant according to claim 23, wherein the passive portions comprisesurfaces provided at a lower end of the channel and covering a majorityof the cross-sectional area of the channel.
 28. A combustion plantaccording to claim 27, wherein the surfaces form an angle of inclinationto a vertical axis of about 20° to 90°.
 29. A combustion plant accordingto claim 16, wherein said combustion chamber and the separating memberare enclosed in a pressure vessel, the combustion plant furthercomprising compressors maintaining a pressure above atmospheric pressurein the pressure vessel.
 30. A combustion plant according to claim 16,wherein the bed has a height between about two meters and six meters.31. A combustion plant according to claim 16, wherein the channelorifice connects to the bed beneath a tube arrangement provided in thebed for heating water and/or superheating steam.
 32. A combustion plantaccording to claim 16, further comprising compressors for feedingoxygen-containing gas at a velocity of 0.5 m/s to 2.0 m/s to the bedthrough nozzles provided beneath the bed.